Influence of filtration on concentrations of 62 elements analysed on crystalline bedrock groundwater samples by ICP-MS

Analyses of unfiltered and filtered (< 0.45 mu m and < 0.10 mu m) groundwat er samples from 15 selected wells in crystalline bedrock aquifers of the Os lo area, Norway, have been studied for 62 chemical elements. While concentr ations of almost all elements vary over several orders of magnitude between the individual wells, the discrepancy between filtered and unfiltered samp les from the same well are rather small, not exceeding one order of magnitu de. Many elements show no influence of filtration at all, while one element (Sn) suggests that filtration may actually introduce contamination to the samples. Correlation between unfiltered and filtered samples is high for mo st elements. The study shows that: (1) even unfiltered samples will satisfa ctorily reflect general water chemistry as long as drinking water (i.e. by definition rather 'clean' water, with low particulates) is collected; (2) f iltered samples do not necessarily reflect 'true' solution chemistry (an el usive concept); and (3) the differences between samples filtered at < 0.45 mu m and < 0.10 mu m are so minimal for most elements, that the additional effort invested in ultra-filtration may not be justified for bedrock ground water samples. (C) 1999 Elsevier Science B.V. All rights reserved.